He’s written for theatre, television and radio, and had poems published in a number of journals. He is a highly acclaimed playwright, with his plays having toured nationally – receiving a number of significant awards for his theatre work. Duncum is also an educator and is the Director of Scriptwriting at the International Institute of Modern Letters, Victoria University.

In the mid 80’s at Victoria University, Duncum won a place on Bill Manhire's creative writing course. Dreams of becoming a poet (he was published as Ken Edlin) were swiftly overtaken by theatre. After writing a number of short plays together, Duncum and student Rebecca Rodden created hit Jism (1988), about siamese twins encouraged to market perfume made from monkey sperm.

Nationwide success followed with Duncum's Blue Sky Boys (1990), in which Michael Galvin and Tim Balme personified dueling musical duo The Everly Brothers. Running parallel to his burgeoning theatrical career, Duncum had began writing for television. In 1988 he contributed sketches to a comedy show made by Wellington's Gibson Group: puppet show Public Eye would lead in turn to Away Laughing, then Skitz.

In the late 90s Duncum joined future thriller writer Donna Malane to plot crime stories for detective John Duggan (John Bach). Tele-movies Duggan: Death in Paradise and Duggan: Sins of the Fathers later spawned a Duggan series. Sins of the Fathers was nominated for a NZ script award. Duncum also wrote for time-travel tale Mirror, Mirror, and joined the scrum of writers on historical potboiler Greenstone.

In September 2001 Duncum received the Michael Hirschfeld Memorial Writing Award. He was also the head writer on hit comedy series Willy Nilly, in which Mark Hadlow and Sean Duffy played mollycoddled farmers, struggling to deal with life without their mother. That year he was appointed director of the MA Scriptwriting programme at Victoria University's Institute of Modern Letters. In 2003, Duncum’s play, Cherish received the award for the Best New, New Zealand Play.

In 2009, Duncum was awarded the Katherine Mansfield Menton Fellowship.